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Re: kern/39206: ffs um_lock handling isn't great
The following reply was made to PR kern/39206; it has been noted by GNATS.
From: David Holland <dholland-bugs%netbsd.org@localhost>
To: Simon Burge <simonb%NetBSD.org@localhost>, gnats-bugs%netbsd.org@localhost
Cc: kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
Subject: Re: kern/39206: ffs um_lock handling isn't great
Date: Fri, 25 Jul 2008 15:24:44 +0000
On Fri, Jul 25, 2008 at 02:15:00PM +0000, Simon Burge wrote:
> One example pointed out by pooka@ is at the top of
> ffs_alloccg(). It appears that once the free block check at
> the top of this function succeeds, this function isn't allowed
> to fail. This is noted in the "XXX fvdl mapsearch ..." comment
> further down. This function is entered with um_lock held, and
> once the free block check has passed um_lock is dropped. This
> then allows another thread to reach the same point, and could
> lead to problems if there was only one block free in the CG
> before the first thread get there.
> This PR is entered as priority "medium" and not "high" since no
> actual problems have been observed in practice yet.
Unless this is the source of those occasional "ffs_alloccg: map
David A. Holland
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